1. Field of the Invention
The present invention relates to thermally developable light-sensitive materials, more particularly, to improved thermally developable light-sensitive materials which comprise thiosulfonic acids therein, whereby generation of heat fog (photographic fog caused in non-exposed areas upon heat development of thermally developable light-sensitive materials, which must be prevented in order to improve the photographic properties of the thermally developable light-sensitive materials) is reduced and freshness retention (the property of maintaining, during storage, photographic properties such as sensitivity and heat fog-preventing properties at the levels immediately after production thereof) is improved.
2. Description of the Prior Art
Various processes are known for preventing heat fogging of thermally developable light-sensitive materials. One comprises using a mercury compound as described in Japanese Patent Publication 11,113/72. However, as is well known, mercury compounds are so toxic that the use thereof in great amounts entails serious dangers. Another process for preventing heat fogging comprises using N-halogeno compounds such as N-halogenosuccinimide, N-halogenoacetamide, etc., as is described in Japanese Patent Publications (OPI) No. 10,724/74, 97,613/74 and 90,118/74. Furthermore, acid stabilizers are known such as higher fatty acids (e.g., lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, etc.) as described in U.S. Pat. No. 3,645,739 and Japanese Patent (OPI) 89,720/73, salicylic acid, p-hydroxybenzoic acid, tetrabromobenzoic acid, tetrachlorobenzoic acid, p-acetamidobenzoic acid, aikyl-substituted benzoic acid (e.g., p-t-butyl benzoic acid, etc.), phthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, diphenic acid, 5', 5'-methylene-bissalicylic acid, and the like. However, these N-halogeno compounds and acid stabilizers do not show marked effects though they possess some heat fog-preventing effect, and therefore, they are commercially unsatisfactory. Furthermore, when used in large amounts, some of these compounds show side effects such as a reduction of sensitivity, deterioration of light stability and a reduction in image density. These compounds are thus of marginal commercial acceptability as well.
At present, the most successful light-sensitive elements capable of forming a photographic image by dry processing are the heat developable light-sensitive elements employing a composition comprising a silver salt of an organic acid, a small amount of a light-sensitive silver halide and a reducing agent, e.g., as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. In such light-sensitive systems, the silver halide which remains in the element after development is not stabilized against light and is allowed to be discolored by light. Nevertheless, the element provides results as if it were stabilized, because the silver halide is only used in a slight amount and most of the silver source is a white or light-colored organic silver salt which is stable and hardly blackened by light. Thus, even if a small amount of silver halide is discolored by light, the slight discoloring does not give rise to any visual difficulties since the element, on the whole, still appears white or only slightly colored. The light-sensitive element is stable at normal temperature, but when it is image-wise exposed and heated, usually above about 80.degree. C., preferably above 100.degree. C., the organic silver salt oxidizing agent and the reducing agent which are present in the light-sensitive element undergo an oxidation-reduction reaction due to the catalytic action of the exposed silver halide present in the vicinity thereof to form silver. By this reaction, the exposed areas of the light-sensitive layer are rapidly blackened so that contrast is formed between the exposed areas and the unexposed areas (background) that is, an image is formed.